Fractionation of sulphur isotopes during ore deposition was studied by analyzing samples of coprecipitated sphalerite, galena and marcasite from four deposits in the Upper Mississippi Valley zinc-lead district. The delta values of S34 for nearly all of the sulphides (main stage of the ore) range from 6.3 per mil to 15.9 per mil relative to the Canyon Diablo standard. The delta values for galena (6.3 per mil to 11.4 per mil) are lower than the delta values for sphalerite (11.4 per mil to 15.9 per mil). The delta values for marcasite, in ore of the main stage, are nearly the same as those for sphalerite. Fractionation factors for coprecipitated sphalerite and galena range from 1.0030 to 1.0073. Fractionation factors for sphalerite and galena from the early part of the ore are smaller than those from the later part of the ore. The fractionation factors for sphalerite and galena when compared to the theoretical fractionation curve of J. R. Hulston give temperatures for sulphide deposition of 150 degrees C to 260 degrees C for the early part of the ore and 100 degrees C or less for the later part of the ore. These temperatures for the later ore are close to temperatures previously determined from fluid inclusions. The temperature and the isotopic composition of sulphur in both the hydrothermal fluid and the minerals appear to have changed at about the same time. The variation found in the isotopic composition of the sulphur from the main stage of ore deposition is almost 10 per mil. More than 7 per mil of this variation is due to fractionation of the isotopes of sulphur in the sulphide minerals, sphalerite and galena.

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